Based on the conventional scheme for the anaerobic digestion of solids, the hydrolysis (solubilization, liquefaction) stage limits the global kinetics of the process. Different physical, chemical and biological pre-treatment processes are applied to the anaerobic digestion to improve the kinetics of the hydrolysis stage and hence improve the kinetics of the global methanogenic process. The thermal hydrolysis process is based on keeping the solid at high temperatures and pressures during relatively long periods, normally longer than 30 minutes. After that, and taking advantage of the high pressure of the hot material, it can undergo a sudden decompression or flash process to achieve the so-called steam explosion effect that fractures the structure of the solids. Other processes utilize heat exchangers to recover energy from the hot material.
Different commercial thermal hydrolysis processes use batch operations. To achieve a nearly continuous process, they operate several reactors in temporized loading and unloading cycles. The well-established CAMBI commercial process operates under those conditions. Patent FR 2820735 includes the use of two batch reactors operating in parallel. Patent WO 2011/006854 A1 “Method and device for thermal hydrolysis and steam explosion of biomass” utilizes three reactors operating in sequential mode to achieve a close-to-continuous flow pattern.
Concerning continuous operation, the Portheous process is applied to digested sludge under anaerobic conditions using reactors with live steam injection, with the objective of improving dewaterability.
Several patents have proposed systems that operate continuously. U.S. Pat. No. 5,593,591 from 1997 claims a system wherein the pumped, pressurized sludge is heated up in the same pipe, before it is decompressed through a nozzle and enters a flash vessel. The objective is to produce a hydrolyzed sludge with good flow features (free flowing solids).
U.S. Pat. No. 5,785,852 from 1998 claims a process and equipment for the treatment of biological sludge to improve the secondary anaerobic digestion. The multi-stage process combines heating, flashing and shear forces application. A “hydrotreater” is the steam and sludge mixing system suggested as the most efficient. The hot, pressurized sludge enters a flash vessel that operates at atmospheric conditions. The partially disaggregated sludge enters a new stage where shear forces are applied to complete the cell structure fracture process.
European patent EP 1 198 424 B1 “A method and arrangement for continuous hydrolysis of organic material” discloses a system consisting of tanks, vessels or recipients wherein the pre-heating, reaction and decompression stages take place continuously. Heating steam injection is carried out in external mixing devices. The sludge residence time in the hydrolysis reactor is fixed between 5 and 60 minutes.
Spanish patent 2 341 064 “Reactor and energy integration system for continuous thermal or thermochemical hydrolysis of organic material” discloses a system that operates continuously and is energy-integrated within the biogas-based electric power generation system.
Patent US2011114570 “An apparatus and a method for continuous thermal hydrolysis of biological material” discloses a tubular reactor wherein the biological solid is heated up with steam injection and is subsequently cooled down with cooling water to avoid flashing during the decompression process.
Patent WO2008/115777 A1 “Treatment of particulate biodegradable organic waste by thermal hydrolysis using condensate recycle” uses an external feed preheater that receives the heating steam. The hot material flows through the tubular reactor and enters a flash vessel, the vapor from the flash chamber is condensed and recycled to the preheat tank.
European Patent application No 13382077.9 “Continuously operating method for the thermal hydrolysis of organic material and installation for implementing the method” is based on the use of internal recycle circuits.
In all the patents analyzed, the hydrolysis reaction is done using tanks, tubular reactors or internal recycle circuits. Those processes indicate that the thermal hydrolysis operation is carried out with long hydraulic residence times.
In all cases, the hot material to be hydrolyzed is compressed using different types of mechanical pumps. The maintenance of those pumps is very demanding, mainly due to the high viscosity and abrasive characteristics of the material to be pumped.